System−Bath Correlation Function Probed by Conventional and Time-Gated Stimulated Photon Echo

Abstract

We show how in the framework of the multimode Brownian oscillator model the system-bath correlation function can be derived from conventional and time-gated stimulated photon echo experiments and consideration of the linear optical spectra. Experiments are performed on the infrared dye DTTCI in room temperature solutions of ethylene glycol, methanol, and acetonitrile. The obtained correlation function is the sum of several Brownian oscillators, of which four are attributed to intrachromophore vibrational dynamics and the other three to solute-solvent dynamics. The ultrafast part of the correlation function on the time scale of the excitation pulses is interpreted as a free induction decay-like effect due to impulsive excitation of spectrally broad underlying vibrational structure in the dye's electronic transition. The slower parts are assigned to multiple time scale solute-solvent dynamics. The effect of vibrational coherences on the echo measurements is also analyzed; this analysis permits the dissection of the correlation function into a part due to intrachromophore dynamics and a part due to solvation dynamics. The spectral densities associated with these latter oscillators are located in the far infrared, in the same spectral region as probed by the optical Kerr effect. The measurements, however, provide no definite answer to the question of whether these spectral densities are the same.

Wave Packet Dynamics in Ultrafast Spectroscopy of the Hydrated Electron

“…83,[85][86][87] Our previous experiments on dyes, however, showed this generally not to be the case. 8 Figure 4 shows that the spectral content of C(ω) agrees rather favorably with the unpolarized Raman spectrum of the highest frequency librational band of neat water 84,88,89 (note that the Raman spectrum of water in Figure 4 is multiplied by ω to allow a direct comparison with the spectral density 91 ). We note that a librational mode of similar frequency was found in model calculations on solvation dynamics in water by Hsu et al 12 We also note that the exponential part of our correlation function is identical to the one reported by Schwartz and Rossky.…”

“…In this model the Stokes shift (the gap between the first moments of emission and absorption) of the electronic transition is 2λ, provided that the excited-state lifetime is large compared to the solvation time. 8 This condition might not be fulfilled for the hydrated electron because of its extremely short excited-state lifetime. The calculations are rigorously carried out taking all possible time orderings and the finite pulse duration into account.…”

<title>Optical coherence spectroscopy in solution: determining the system-bath correlation function</title>

“…The Fleming and Wiersma groups have argued that one measurement, the photon echo peak shift (PEPS), strongly reflects the correlation function M(t). 4,5 A PEPS measurement involves taking a series of photon echo measurements for different fixed values of T 23 . Each photon echo measurement has a signal maximum at a particular value of T 12 .…”

“…This has already been shown for certain carbocyanine dye molecules. 4,5 In this paper, we test this assumption for bacteriochloropyll a . …”

“…Several groups have taken advantage of these ultrashort pulses to perform third-order optical coherence measurements of chromophores dissolved in solvents at room temperature. 3,4,5 In parallel to these experimental developments, Mukamel and co-workers have developed a theoretical picture, based on time correlation functions for the energy gap between optical states, that gives insight into the dynamical processes that underlie the experimental data. 6 This paper describes a method to extract correlation functions that reveal the dynamics of a chromophore in liquid solution from optical coherence measurements.…”